Ontology

Ontology is the philosophical study of the nature being, of becoming, of existence, or reality, as well as the basic category of being in their relations. Ontology has been considered to be a part of, or even equivalent to, metaphysics, with the terms being used almost interchangeably.

History
Ontology is the examination of everything entailed in the word "being" or the word "thing". Some philosophers, including Platonists, argue that all nouns (including abstract nouns) refer to existent entities, while some others argue against this, arguing that geometry is a collection of intellectual activities, and is not itself an existent entity.

The fundamental questions in ontology would be such questions as, "What can be said to exist?", "What is a thing?", and "What are the meanings/modes of being?". A common approach involves dividing extant subjects and predicates into categories, with these categories differing widely. Aristotle's ten categories endured the test of time, but philosophers like Immanuel Kant came up with their own categories. Aristotle's categories deal with the essence of a thing, quality, quantity, relativity, and other major ontological issues. Ontology also questions what "being" or "existence" is, and this is of vital importance regarding a renowned argument used from the Middle Ages to prove God's existence. Others can question whether existence is divided into species, what entities (if any) are fundamental, if all entities are objects, if physical properties actually exist, what features are essential to given objects, how many levels of existence are there, what constitutes a level for that matter, what is a physical object, what does it mean to say that a physical/non-physical entity exists, when does an object go out of existence as opposed to merely changing, or if the subject-object split inevitable. Ontology relates to many other fields that might not have a direct connection.

Dichotomies
In ontology, there are a number of common dichotomies, seven of which are: universals (repeatable entities, common to many) and particulars ; substance (oucia, with the first being an individual {such as a human}, and the second being possessive of substantiality, while only existing in a derivative sense {such as humanity}) and accidents (attributes of a certain substance {hair color, weight, etc.}, existing in another, and not in itself); abstract  (has no physical reference) and concrete  (has a physical reference);  essence  (confers quiddity {whatness}; contrasted with accident, which does not determine "whatness"; to ti esti {what it is}; opposed by nominalists) and  existence ;  determinism  (all events are determined by previous causes) and  indeterminism  (no event is certain, and the outcome of anything is probabilistic);  monism  (all existing things go back to a single source; only one thing is prior to everything else) and  dualism  (co-eternal binary opposition; everything is divided in two; body and spirit, matter and mind, good and evil, light and dark, etc.); and  idealism  (reality is fundamentally mental and immaterial, conscience determines material and is the origin of the world, all entities are composed of mind and spirit) and  materialism  (material determines conscience).